A contact probe of this type typically comprises a holder in the form of a plate member defining a plurality of holder holes passed across the thickness of the holder, a compression coil spring received in each holder hole and a pair of needle members connected to either axial end of the compression coil spring. An electroconductive contact probe of this type is typically used between an object to be tested and a circuit board of a testing device.
In case of a contact probe having two moveable ends, the two needle members for each coil spring project from the corresponding sides of the holder and are prevented from coming out beyond a certain limit by shoulders or the like formed in the holder. In case of a contact probe having only one moveable end, one of the needle member is prevented from coming off in a similar manner while the other needle member abuts corresponding pad of a circuit board which is attached to the corresponding side of the holder.
In either case, to the end of obtaining a favorable resilient force from each coil spring, the coil spring is installed in a prestressed state. By so doing, changes in the resilient force in relation with the retracting stroke of the corresponding needle member can be minimized.
However, the inventor has discovered that, as the number of contact units (each including a compression coil spring and a pair of needle members provided on either axial end of the coil spring) in each contact probe increases to meet the need to test highly densely arranged terminals or pads of modern semiconductor devices, the combined spring force of the coil springs in the contact probe may become so great that it becomes increasingly difficult to provide an adequate mechanical strength to the holder. Because each contact unit is very small, the spring force of each contact unit may be extremely small, but the number of contact units is so great in some of the contact probes for testing advanced semiconductor devices that the combined spring force may become very significant.
In particular, because at least one of the needle members of each contact unit is required to be prevented from coming off by a shoulder or the like formed in the holder hole, such a shoulder or the like is constantly subjected to the preloading of the coil spring. Thus, the combined spring load acts upon the shoulder and tends to push one part of the holder away from another.
When the compression load of the coil springs is large enough, the holder member typically made by a plurality of layered support members subjected to this compression load may warp, deflect or otherwise deform. This would give rise to such problems as the loss of precision in the position of the free ends of the needle members (contact positions) and an increased resistance to the movement of the needle members. Such problems could be avoided by increasing the thickness of the corresponding support member.
However, as the frequency of the signal for testing the test objects becomes higher, the contact probe is required to be adapted to such high frequency signals. For instance, the total length (the length of the path for the test signal) is desired to be minimized. This can be accomplished by reducing the total thickness of the holder (axial length of each electroconductive contact unit) by reducing the thickness of each support member, but the reduced thickness of the support member means a reduced mechanical strength, and compounds the problem.
When a burn-in test is conducted on a semiconductor device by applying a voltage for a prolonged period of time (from a few hours to tens of hours) at a high temperature environment (approximately 150° C.), it is desirable to use an insulating material having a relatively low coefficient of thermal expansion such as ceramics for the support members. However, when the support members are made of brittle material such as ceramics, they could be damaged when the spring load is great. Therefore, in some cases, ceramic material cannot be used for the support members of an electroconductive contact probe which is required to have a small thickness. Plastic material may be used for the support members when the contact unit is intended to be used in a room temperature environment, but the support members may deform under the load even in such cases.